Many bioactive compounds are highly lipophilic (hydrophobic), meaning that they are soluble in lipids (oils) and some organic solvents, while being substantially insoluble or only sparsely soluble in water. The lack of solubility of a bioactive compound in aqueous media is an important factor limiting its therapeutic applications, making difficult an efficient administration of the compound to a patient. When administered in the form of an oil solution or some kind of water and/or oil suspension or emulsion, lipophilic compounds usually show a poor bioavailability, meaning a low concentration and a long build-up time of the compound in the systemic circulation. This lack of bioavailability is usually independent of the administration route (topical, oral, or parenteral).
Various approaches to overcoming this limitation are known in the prior art. One known approach consists of dissolving a lipophilic compound in a water-miscible organic solvent, such as ethanol or propylene glycol. When such a solution is admixed with blood or gastrointestinal fluids, however, the lipophilic compound usually precipitates as a solid or liquid emulsion, with a resulting low bioavailability. Furthermore, for many lipophilic compounds no organic, water-miscible solvents exist. Another approach consists of incorporating lipophilic compounds into various compositions, frequently inhomogeneous, multiphase emulsions, containing oils and solvents in combination with surfactants. These compositions may improve the bioavailability of the compound without significantly increasing its solubility in aqueous media, but are normally suitable only for a particular administration form, usually for topical applications. Such compositions, which may also induce a protective immune response in mammals, are of little value for therapeutic uses where administration of the compound by ingestion or injection is necessary and where an aqueous solution or a water-soluble solid composition is frequently the only acceptable administration form.
Several approaches to preparing homogenous aqueous solutions of lipophilic bioactive compounds are also known in the prior art. One method consists of preparing a derivative or an analog of a lipophilic compound having a better solubility in water than the original compound. In the simplest case, this derivative may be a water-soluble salt of the compound, which salt usually retains the original biological activity, but this approach is applicable only to compounds having acidic or basic properties. If more substantial modifications are introduced into the original compound to improve its solubility, a decrease or even a complete loss of the original bioactivity of the compound is frequently observed.
Another method of solubilization consists of preparing a water-soluble derivative capable of liberating the original bioactive compound under physiological conditions. Such derivatives, also known as pro-drugs, usually improve bioavailability of the compound and may also ensure a targeted delivery of the compound or its sustained release over a period of time. However, this approach usually relies on the presence of certain functional groups in the original compound, so it is not universally applicable. In addition, synthetic methods of improving solubility of a compound by chemical modifications are relatively complicated and expensive.
Still another approach to solubilization of bioactive lipophilic compounds relies on formation of water-soluble complexes. An example are complexes with amphipathic compounds containing moieties of two opposing solubility tendencies (lipophilic and hydrophilic). Such compounds are often soluble both in organic solvents and in water, so that the solubilization is usually achieved by dissolving a bioactive lipophilic compound and an amphipathic compound in a suitable water-miscible organic solvent and diluting the solution with water. In some cases the organic solvent is partially or entirely removed from the original or water-diluted solution by evaporation or lyophilization and the concentrate reconstituted with a suitable aqueous medium, without precipitation of the water-insoluble lipophilic compound. When the auxiliary organic solvent cannot be completely removed from the composition, this solvent must be pharmaceutically acceptable, which limits the choice of applicable solvents.
Bioactive lipophilic compounds in need of solubilization belong to various therapeutic categories, such as vitamins (e.g., Vitamin E), antibiotics, in particular macrolide polyene antibiotics (amphotericin-B, nystatin, candicidin), free radicals scavengers (e.g., tocopherols, ubiquinones), immunosuppressants (e.g., cyclosporine), etc. Various approaches to achieve the solubility and improve the bioavailability of these and other lipophilic compounds are known in the prior art, including formation of water-soluble complexes.
U.S. Pat. No. 5,686,110 discloses water-soluble complexes of water-insoluble compounds, including amphotericin-B and cyclosporine, with a polyalkylene oxide polymers end-capped with an alkyl or olefinic group, which polymers are soluble both in water and in organic solvents. The water-soluble complexes, which are formed only in the presence of an auxiliary organic solvent, are lyophilized and reconstituted with a buffer solution. The reconstituted aqueous solutions show only a limited stability, depending mostly on the pH of the solution. Furthermore, the use of methoxypolyethylene glycol polymers of relatively high molecular weight (2,000 to 5,000) as preferred solubilizing agents increases the amount by weight of polymer necessary for solubilization of the bioactive compound.
U.S. Pat. No 5,798,333 discloses a homogenous, water-soluble composition (concentrate) of cyclosporine, which can be diluted with an aqueous solution without precipitation of cyclosporine. The concentrate comprises cyclosporine and tocophersolan (polyoxyethanyl-a-tocopheryl succinate, TPGS) dissolved in a hydrophilic organic solvent, such as propylene glycol. Solvent-free compositions are not disclosed, as they would likely be unstable or in homogeneous.
WO 96/17626 discloses water-soluble compositions of ubiquinones comprising polyoxyethanyl-cholesteryl sebacate (PCS) as a solubilizing agent. A ubiquinone, in particular Coenzyme Q.sub.10, is solubilized by dissolving both Coenzyme Q.sub.10 and PCS in tetrahydrofuran at an approximate molar ratio of 1:3 and diluting this solution with water. The solution is then evaporated to dryness under reduced pressure and reconstituted with a suitable buffer solution.
Coenzyme Q.sub.10 (CoQ10) is a natural compound whose therapeutic potential has been recently recognized for a number of disorders, including congestive heart failure, muscular distrophy, periodontal disease, correction of drug-induced deficiencies, and immune restoration (AIDS, allergies), to name a few. Coenzyme Q.sub.10 is also of great interest to cosmetic industry, since it can be included into cosmetic preparations as agent slowing down natural skin ageing processes. The biological activity of Coenzyme Q.sub.10 is believed to be linked to its ability to act as an antioxidant and free radical scavenger protecting integrity of cell membranes and to offset the inability of diseased cells to manufacture sufficient energy for cellular repair, by stimulating mitochondrial respiration and production of ATP. For effectiveness in both clinical and cosmetic applications, preparations of Coenzyme Q.sub.10 with high bioavailability and solubility in aqueous media are usually required.
Even though various methods of improving solubility of lipophilic compounds, such as Coenzyme Q.sub.10, in aqueous media are known in the prior art, they are not equal in terms of simplicity, scope of applicability, stability of the prepared formulations, etc. The present invention provides a new method of solubilization of lipophilic compounds, which is free of many prior art drawbacks and limitations.